This invention relates generally to fuel vapor recovery systems and more particularly to a pulse width modulated solenoid purge valve for an on board fuel vapor recovery system of an automotive vehicle.
An on board fuel vapor recovery system employs a charcoal filled vapor canister that stores fuel vapor vented from the fuel tank of the automotive vehicle. The canister is connected to the intake manifold of the vehicle engine so that the stored vapor is drawn into the engine and combusted during engine operation.
The rate at which stored vapor is drawn into the vehicle engine for combustion must be controlled precisely to avoid overly enriched fuel mixtures as controlled by the carburetor, fuel injectors or other charge forming devices of the vehicle engine. In order to control flow from the storage canister to the intake manifold in a precise manner, on board fuel vapor recovery systems employ a purge valve.
Purge valves for an on board fuel vapor recovery system are commonly operated by a solenoid that is under the control of a computer programmed to open and close the valve at an intermittent rate determined by various operating characteristics monitored by the computer. Essentially, the computer functions to open the valve at a cyclic frequency, which may be varied by the computer, for a selected portion of each cycle (the duty cycle), which may also be varied by the computer. Thus the typical control strategy for a solenoid operated purge valve comprises energizing the coil of the solenoid by a pulse width modulated signal from an electrical circuit that is controlled by the programmed computer. The extent to which the solenoid operated purge valve opens is related to the duty cycle of the modulation.
In the past these pulse width modulated solenoid purge valves included a pole piece and an armature that had flat confronting faces at an air gap that separated them. See for instance, U.S. Pat. No. 4,944,276 granted to William J. House et al Jul. 31, 1990. The air gap shape configuration typified by the House patent produces a relatively slow response time of the armature when the coil of the solenoid is energized. Consequently, these known pulse width modulated solenoid purge valves are operated at a relatively low frequency of about 5 Hertz or cycles per second.
U.S. Pat. No. 5,237,980 granted to William C. Gillier Aug. 24, 1993 discloses an on-board fuel vapor recovery system having a pulse width modulated solenoid purge valve. This purge valve has a step incorporated into the confronting faces of the armature and pole piece at their interface across the air gap. According to the Gillier patent, this step assists in damping the motion of the armature to cure an irregularity that might otherwise be present in the flow characteristic of the system. This step however, does not decrease the response time of the armature significantly. Consequently, the purge valve disclosed in this patent is still operated at a relatively low frequency.